WHO picks two new strains for next season's flu vaccine

Feb 23, 2012 (CIDRAP News) – The World Health Organization (WHO) today recommended changing two of the three strains in next season's influenza vaccine for the Northern Hemisphere, ending a couple of years of stability in the vaccine's makeup.

The WHO's flu experts called for changing the A/H3N2 and B components of the vaccine but continuing to use a strain of the pandemic 2009 virus for the A/H1N1 component.

Although the current flu season has been quiet in most of the world, the recommendation means that a significant share of flu viruses circulating in recent months don't match up very closely with those in this year's vaccine.

The WHO recommended strains similar to the following for the 2012-13 season:

A/California/7/2009 (H1N1)pdm09

A/Victoria/361/2011 (H3N2), which replaces A/Perth/16/2009

B/Wisconsin/1/2010, which replaces B/Brisbane/60/2008

The WHO experts make their recommendations for the North Hemisphere flu vaccines in February to allow time to prepare the vaccine viruses and grow them in eggs, which takes several months. The recommendation for the Southern Hemisphere vaccine is usually made in October.

The strain changes announced by the WHO appear to be the first for the Northern Hemisphere vaccine since February 2010, when the agency recommended adding the 2009 pandemic strain to the 2010-11 vaccine.

The WHO's technical report says that since September, 2009 H1N1 viruses have circulated at very low levels in most countries, and most of the isolates studied have been similar to the vaccine strain.

H3N2 viruses have been predominant in Europe, many countries in the Americas and northern Africa, and some Asian countries. The majority of recent H3N2 isolates have been distinguishable from the current vaccine strain and more closely related to A/Victoria/361/2011-like reference viruses, the report states.

Influenza B viruses come in two lineages, Victoria and Yamagata, and predicting which one will be more common in any given season has been difficult.. The WHO's choice for next season's B strain marks a switch from Victoria to Yamagata.

The report said the proportion of B/Yamagata/16/88-lineage viruses has increased this season in many countries, though Victoria lineage viruses have predominated in some countries, including China. Most recent Yamagata isolates have been distinguishable from the previous Yamagata strain used in the vaccine (B/Florida/4/2006) and are closely related to B/Wisonsin/1/2010-like viruses.

The WHO report also notes that 21 human cases of H5N1 avian flu were reported from Sep 20, 2011, to Feb 21, 2012, of which 15 were fatal. They occurred in Cambodia, China, Egypt, Indonesia, and Vietnam.

The report also notes the handful of cases of novel swine-origin H3N2 infections in the United States in late 2011. No human infection with H9N2, an avian strain that sometimes infects humans, have been reported in recent months, the WHO says.